In fog the conspicuity of a vehicle may be increased by using high-intensity rear lights. However, these lights can be much brighter than the vehicle's brake lights and may mask them if they are installed too close to them. An experiment to investigate the effect of increasing the separation between these lights and the brake lights was conducted in artificial fog at TRRL. A rear light display of two brake lights and two high-intensity rear lights was situated in the peripheral visual field of the subject who had to react to the brake light onsets. At the same time subjects performed a second task, demanding similar concentration to driving, which also simulated a driver being distracted from looking straight ahead. Subjects reacted more quickly and missed fewer presentations as the separation between the brake lights and high-intensity rear lights was increased. The greatest improvement in performance occurred when the angular separation, between the lights as measured from the subject's view-point, was changed from zero (the lamps touching) to 5.7 minutes of arc with smaller further benefits occurring with angular increases up to 17 minutes of arc. It is suggested that a minimum separation between the different types of lamp of 100 mm would be the most suitable since this would meet the 5.7 minute of arc criterion during following at high speed in thin fog and would exceed this at the shorter viewing distances likely in thicker fogs. (A)

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